1. Field of the Invention
This invention involves a method and an apparatus for converting any M gray levels in a K-gray scale (halftone) image to M gray levels in an N-gray scale image, where K&gt;N.gtoreq.M.
More particularly, the invention is directed to a method and an apparatus whereby image display output data from a personal computer, or the like, adapted to be displayed on one device, such as a CRT capable of displaying an image with K (e.g., 64)-gray scale levels, may also be displayed on another device, such as a plasma display capable of displaying an N (e.g., 16)-gray scale level image, in such manner that M (e.g., 14) gray levels in the K-gray scale level image are selected and converted to the N-gray scale level image so as to be mutually identifiable.
2. Prior Art
It is known that a dot matrix image having a multi-level gray scale may be displayed in a halftone representation using a dithering technique in order to convert it for display with another multi-level gray scale. However, with this technique, one-dot gray scale is converted to a multi-dot halftone and it cannot be applied to a one-dot to one-dot conversion.
Some personal computers can display a multi-level gray scale representation on a CRT, but they are not adaptable to convert that gray scale representation to another type of multi-level gray scale representation. Also, since the output from a personal computer to a printer is usually a two-level gray scale output, white and black, a gray scale conversion capability from one multi-level gray scale to some other multi-level gray scale is not provided.
At present there are a number of different display devices with various gray scale capabilities. For instance, an output device such as a CRT may display an image with, for example, a 64-gray scale representation while another display device, such as a conventional plasma display panel (PDP), can only display an image in, for example, a 16-gray scale. Consequently, it is necessary to convert display data outputted from the CRT controller for a 64-gray scale display to adapt it for display in 16-gray scale; or, in other words, if M (M.ltoreq.16) gray levels in the 64 gray scale outputted from the CRT controller are converted to ones in a 16-gray scale representation, they can be displayed on the PDP. However, if the 64-gray scale representation is converted to a 16-gray scale representation on a proportional distribution basis, that is, for example, if the conversion to the 16-gray scale representation is carried out by extracting the upper y (e.g., 4) bits of the 64-gray scale x (e.g., 6) bit representation (hereinafter such a conversion is referred to as a "natural conversion"), degeneration of the gray levels can occur wherein some gray levels that are different from each other in the 64-gray scale representation are represented as the same gray level in the 16-gray scale representation. Such degeneration of gray levels presents a problem because gray levels which were originally identifiable become unidentifiable and the image represented may appear quite different on different displays.
It is thus desirable, and the object of the present invention, to solve this problem by converting first gray level signals to second gray level signals in such manner that, when each of M gray levels represented in a K-gray scale image is converted to a level represented in an N-gray scale image, the conversion is made without causing the degeneration of the gray levels and in accordance with the relative sequence and magnitude of their brightnesses.